= Solution
The left panel below shows paddles in a bath connected by an axle to an asymmetric toothed wheel, a spring-loaded pawl and a load. A naive argument lets random paddle kicks turn the wheel in one direction while the pawl blocks reverse motion, apparently extracting work from a single bath. The missing ingredient is the pawl's own <thermal fluctuations>. At the same <temperature>, it sometimes lifts and allows reverse slips. Correct forward and reverse transition rates obey <detailed balance>, so no sustained directed rotation can do work at single-bath <thermal equilibrium>. The device illustrates, rather than disproves, the <Second law of thermodynamics>.
A macroscopic shaken ratchet is driven far from <thermal equilibrium>: imposed shaking supplies organized mechanical <energy>, and friction or inelastic collisions dissipate it. Its kilogram-scale parts are not moved appreciably by molecular <thermal fluctuations>. Rectification requires broken spatial symmetry from the teeth or pawl and appropriate dynamical coupling: the shaking must sometimes disengage or drive the pawl over a tooth while contact and dissipation prevent equal reverse motion. The load must remain below the available mean driving <torque>. Isotropic, aperiodic shaking can satisfy these conditions because the asymmetric mechanism breaks directional symmetry; without the external <energy> supply, a working rectifier and dissipation, there is no guaranteed directed motion.
\Image[/past-exam-of-the-short-options-paper-shared-by-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2019-biophysical-sketches.png]
{title=Original schematics of a thermal ratchet, rotary ATP synthase and a diffraction-limited point image}
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